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One particularly monstrous black hole has probably been humming a B flat note for billions of years, although at a pitch so low that no human could hear it - even if sound could travel through space.

Professor Andrew Fabian of the Institute of Astronomy at Cambridge, England and colleagues base their claim on data collected by NASA's orbiting Chandra X-Ray Observatory. They investigated X-rays coming from the heart of the Perseus Cluster, a giant clump of galaxies some 250 million light-years from Earth.

"The intensity of the sound is comparable to human speech," sai Fabian. But the pitch of the sound is about 57 octaves below middle C - which is at roughly the middle of a standard piano keyboard. This is far, far deeper than humans can hear and the researchers believe it is the deepest note ever produced by the universe.

Researchers presume that a supermassive black hole, with perhaps 2.5 billion times the mass of our Sun, lies at the centre of Perseus. Black holes are powerful matter-sucking drains in space, and astronomers believe most galaxies, including our own Milky Way, may contain black holes at their centers. Black holes have not been directly observed, because their gravitational pull is so strong that nothing, not even light, can escape it.

So researchers have concentrated on what happens around the edges of black holes, just before matter is pulled in. When scientists trained the Chandra observatory on the center of Perseus last year, they saw concentric ripples in the cosmic gas that fills the space between the galaxies in the cluster.

"We're dealing with enormous scales here," Fabian said. "The size of these ripples is 30,000 light-years." A light-year is about 10 trillion km, the distance light travels in a year.

He said the ripples were caused by the rhythmic squeezing and heating of the cosmic gas by the intense gravitational pressure of the jumble of galaxies packed together in the cluster. As the black hole pulls material in, he said, it also creates jets of material shooting out above and below it, and it is these powerful jets that create the pressure that creates the sound waves.

To scientists, he said, pressure ripples equate to sound waves. By calculating how far apart the ripples were, and how fast sound might travel there, the team of researchers determined the musical note of the sound.

Fabian said the notion of singing black holes might well be extrapolated to other galaxies, but not necessarily to the Milky Way.

Chandra has looked at X-ray emissions from the Milky Way's center, and astronomers believe there is a black hole there, but because it is a young, rambunctious galaxy with lots of activity at its heart, this may interfere with any note our black hole might sing, Fabian said.